Ectoparasite infestation gives rise to various immuno-allergic reactions in the host, and may also cause development of resistance. Occurrence of the phenomena more or less simultaneously has led to a belief that resistance is related to the immuno-allergic reactions but, in fact, there is little evidence to support the assumption. We have been studying the histopathology of innately susceptible and resistant animals to initial and repeated exposures to ectoparasites to try to identify the reactions of resistance. Intermittent and continuous exposure of animals to ectoparasites during the neonatal period has been done to determine whether tolerance can be induced. After extensive trials there is no evidence of it.